rear pressure low in system HELP

I installed a Right Stuff kit in the car and I have a disc on the rear. 1969 Plymouth Valiant.

I'm not getting enough fluid pressure to the rear disc callipers to have much of an effect. Engine running and hard on the brake peddle the rear tires still turn in drive. I have bled the system several times and there is no air left in the system. I have tried the system with the manual proportioning valve all the way open and all the way close and several settings in between. I think what is happening is that the front brake circuit (rear reservoir) is building up pressure and stopping the master cylinder piston from traveling further in the bore to build up pressure in the rear brake circuit (front reservoir). The line lock valve is not connected. There is no stock proportioning valve in the system.

I'm stumped on this one and need your advice.

Thank you in advance.:prayer: